Hydraulic fluids with improved anti-corrosion properties

ABSTRACT

Hydraulic fluids, in particular brake fluids for motor vehicles, having improved corrosion protection, comprising  
     (a) from 0.005 to 0.125% by weight of 1H-1,2,4-triazole, and  
     (b) from 0 to 10% by weight of one or more further corrosion inhibitors, where, on concomitant use of 1H-1,2,3-benzotriazole and/or 1H-1,2,3-tolutriazole and/or derivatives thereof, the weight ratio between 1H-1,2,4-triazole and the said 1H-1,2,3-triazoles must be greater than 4:1.

[0001] The present invention relates to hydraulic fluids, in particular brake fluids for motor vehicles, having improved corrosion protection which comprise

[0002] (a) from 0.005 to 0.125% by weight of 1H-1,2,4-triazole, and

[0003] (b) from 0 to 10% by weight of one or more further corrosion inhibitors, where, on concomitant use of 1H-1,2,3-benzotriazole and/or 1H-1,2,3-tolutriazole and/or derivatives thereof, the weight ratio between 1H-1,2,4-triazole and said 1H-1,2,3-triazoles must be greater than 4:1.

[0004] Hydraulic fluids and in particular brake fluids for motor vehicles are subject to very high requirements with respect to their chemical and physical properties. In accordance with the existing standards and specifications for brake fluids from the US Department of Transportation in Federal Motor Vehicle Safety Standard FMVSS-No. 116 and the SAE J 1704 standard published by the Society of Automotive Engineers, modern brake fluids should on the one hand have high dry boiling points (equilibrium reflux boiling points “ERBP”) and high wet boiling points (“wet ERBP”), but on the other hand should also have a viscosity which changes only slightly within a broad temperature range.

[0005] In addition, there are more exacting requirements from the automobile industry for improved corrosion protection in the case of metals and nonferrous metals. Corrosion inhibitors in brake fluids have the job of preventing destruction of metallic materials caused by corrosion.

[0006] The use of 1H-1,2,3-triazoles, such as 1H-1,2,3-benzotriazole or 1H-1,2,3-tolutriazole, as corrosion inhibitors in hydraulic fluids or brake fluids has been known for some time, for example from EP-A 028 789 (1) and EP-A 454 110 (2).

[0007] WO 00/46325 (3) recommends hydraulic fluids which comprise a combination of 1H-1,2,3-benzotriazole or derivatives thereof, such as 1H-1,2,3-tolutriazole, and 1H-1,2,4-triazole or derivatives thereof in a weight ratio of from 1:4 to 4:1 as corrosion inhibitor system. For comparative purposes, Tables 1 to 4 in (3) also indicate hydraulic fluids which comprise 1H-1,2,4-triazole alone in concentrations of 0.15% by weight; however, its corrosion-inhibiting action is acceptable only in the case of copper.

[0008] The hydraulic fluids and brake fluids for motor vehicles which are known in the prior art are, however, still unsatisfactory with respect to their properties and in particular their corrosion protection. It was therefore an object of the present invention to provide hydraulic fluids which, in particular, satisfy the abovementioned requirements for improved corrosion protection in the case of metals and nonferrous metals.

[0009] We have found that this object is achieved by the hydraulic or power-transmission fluids defined at the outset.

[0010] The hydraulic fluids, in particular motor vehicle brake fluids, according to the invention preferably comprise from 0.005 to 0.10% by weight, preferably from 0.005 to 0.08% by weight, in particular from 0.005 to 0.06% by weight, of component (a).

[0011] In a preferred embodiment, the hydraulic fluids, in particular motor vehicle brake fluids, according to the invention comprise component (a) as the only triazole-based corrosion inhibitor. This is intended to mean that triazoles, such as 1H-1,2,3-benzotriazole, 1H-1,2,3-tolutriazole or hydrogenated 1H-1,2,3-tolutriazole, are excluded here, but inorganic and/or organic corrosion inhibitors having a different structure may be present.

[0012] In another preferred embodiment, the hydraulic fluids, in particular motor vehicle brake fluids, according to the invention also comprise up to 10% by weight, in particular up to 5% by weight, of one or more further corrosion inhibitors (b), where, in the case of the concomitant use of 1H-1,2,3-benzotriazole and/or 1H-1,2,3-tolutriazole and/or derivatives thereof, the weight ratio between 1H-1,2,4-triazole and said 1H-1,2,3-triazoles must be greater than 4:1.

[0013] Suitable further corrosion inhibitors (b) here are, in particular, alkali metal salts of phosphoric acid and phosphorous acid, fatty acids, such as caprylic, lauric, palmitic, stearic or oleic acid and alkali metal salts thereof, esters of phosphoric acid and phosphorous acid, such as ethyl phosphate, dimethyl phosphate, isopropyl phosphate, diisopropyl phosphate, butyl phosphite or dimethyl phosphite, substituted or unsubstituted ethoxylated mono- and dialkylamines and salts thereof with mineral acids and fatty acids, for example butylamine, hexylamine, octylamine, isononylamine, oleylamine, dipropylamine, diisopropylamine or dibutylamine, substituted or unsubstituted ethoxylated alkanolamines, for example mono-, di- or triethanolamine, N,N′-di-n-butylaminoethanol or 1,1′-iminodipropan-2-ol, cyclohexylamine, benzimidazole and hydrogenated tolutriazole, and nitroaromatic compounds, for example 3-nitrobenzaldehyde.

[0014] Said corrosion inhibitors (b) are preferably present in the hydraulic fluids, in particular motor vehicle brake fluids, according to the invention in amounts of from 0.001 to 10% by weight, in particular from 0.001 to 5% by weight.

[0015] In a particularly preferred embodiment, the hydraulic fluids, in particular motor vehicle brake fluids, according to the invention comprise, besides 1H-1,2,4-triazole (a), from 0.001 to 5% by weight, in particular from 0.001 to 0.5% by weight, of one or more compounds from the group consisting of benzimidazole, hydrogenated 1H-1,2,3-tolutriazole, purine, adenine, 6-chloropurine, 2,6-dichloropurine, 6-methoxypurine, 1H-1,2,3-triazolo(4,5-b)pyridine, 6-histaminopurine and 6-furfurylaminopurine and, if desired, further corrosion inhibitors other than (a) as additional corrosion inhibitors (b). Said purine derivatives are described in German Patent Application P 100 26 010.1 as non-ferrous metal corrosion inhibitors for hydraulic fluids or brake fluids.

[0016] All the percent by weight data given above and below in each case relate to the total amount of the hydraulic fluid or brake fluid, unless stated otherwise. The sum of all components listed above and below for the hydraulic fluids or brake fluids according to the invention in each case adds up to 100% by weight.

[0017] The present invention relates, in particular, to brake fluids for motor vehicles which comprise, as corrosion inhibitors, said components (a) and, if desired, (b).

[0018] In a preferred embodiment, the motor vehicle brake fluids according to the invention furthermore comprise, in addition to components (a) and, if desired, (b), from 0.1 to 97% by weight, in particular from 30 to 97% by weight, especially from 50 to 97% by weight, of one or more polyglycol ethers and/or borates thereof.

[0019] Suitable polyglycol ethers here are, in particular, ethylene glycol monoalkyl ethers having up to 6 ethylene oxide units and having up to 4 carbon atoms in the alkyl radical, for example diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, triethylene glycol monobutyl ether or tetraethylene glycol monomethyl ether. Also suitable are ethylene glycol dialkyl ethers and propylene glycol dialkyl ethers having up to 6 alkylene oxide units and having up to 4 carbon atoms in each of the alkyl radicals, for example triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, diethylene glycol di-tert-butyl ether, diethylene glycol dibutyl ether, diethylene glycol di-tert-butyl ether or dipropylene glycol dimethyl ether.

[0020] Suitable boric acid esters of said or other polyglycol ethers are described, in particular, in the specifications EP-B 013 925 (cyclic bisboric acid esters), DE-C 28 04 535 (nitrogen-containing boric acid esters), DE-A 24 38 038 (boric acid alkylene glycol monoalkyl ether esters) and DE-B 17 68 933 (boric acid trisalkoxyalkyl esters).

[0021] Instead of said polyglycol ethers and/or boric acid esters thereof, the motor vehicle brake fluids according to the invention may also comprise, as principal component, those based on carboxylic acid esters, mineral oils or silicone oils.

[0022] In a further preferred embodiment, the motor vehicle brake fluids according to the invention furthermore comprise, besides components (a) and, if desired, (b), from 0.1 to 50% by weight, in particular from 1 to 40% by weight, especially from 5 to 30% by weight, of one or more polyglycols.

[0023] Suitable polyglycols here are, in particular, relatively high-boiling products of the reaction of ethylene oxide and/or propylene oxide and/or butylene oxide with water or diols, in particular corresponding products of the mixtures of ethylene oxide and propylene oxide with water. The number of alkylene oxide units in polyglycols of this type is normally from 2 to 10.

[0024] The action of these high-boiling polyglycols is that of a lubricant, which is attributable essentially to an improvement in the temperature/viscosity behavior. The polyglycols give the low-viscosity polyglycol ethers sufficient viscosity at high temperatures and thus ensure adequate lubrication. Adequate lubrication is necessary in the components of the motor vehicle braking system since rubber or elastomers have to slide on metal with very low wear in these systems.

[0025] Further components and assistants in the motor vehicle brake fluids according to the invention can be conventional antioxidants, for example phenothiazine and/or those based on phenol, and conventional antifoams.

[0026] In addition, the brake fluids according to the invention may comprise the cyclic carboxylic acid derivatives mentioned in WO 00/65001, which are suitable as components for reducing the low-temperature viscosity in the presence of water.

[0027] The presence of said small amount of 1H-1,2,4-triazole ensures, in an excellent manner, that the hydraulic fluids or motor vehicle brake fluids according to the invention meet the requirements mentioned at the outset and that, in particular, a corrosion behavior which is generally good compared with the prior art, i.e. improved corrosion protection in the case of metals, such as iron, steel, tinplate, cast iron (gray iron), lead, tin, chromium, zinc, aluminum, magnesium and alloys thereof, and in the case of solder metals, for example solder tin, and in the case of non-ferrous metals, such as copper and alloys thereof, for example brass, is effected.

[0028] Further advantages of the hydraulic fluids or motor vehicle brake fluids according to the invention are their favorable low-temperature viscosity, good water compatibility, a mild pH, good low-temperature, high-temperature and oxidation stability and good chemical stability, a favorable behavior (i.e. good compatibility) with materials such as rubbers, plastics, glue lines, fiber, elastomer and rubber seals and similar materials, as well as good lubricating behavior.

[0029] Not least, the use of only a small amount of 1H-1,2,4-triazole in the hydraulic fluids or motor vehicle brake fluids according to the invention is advantageous both in economic and in ecological terms.

[0030] The examples below are intended to explain the present invention without representing a restriction.

APPLICATIONAL EXAMPLES

[0031] Based on the commercially available DOT 4 or DOT 5.1 brake fluids BF 1 (ERBP: 267° C., wet ERBP: 173° C., kinematic viscosity at −40° C.: 676 cSt) and BF 2 (ERBP: 263° C., wet ERBP: 181° C., kinematic viscosity at −40° C.: 850 cSt) each with 0.05% by weight of 1H-1,2,3-tolutriazole as non-ferrous metal corrosion inhibitor, the brake fluids BF 1a, BF 1b, BF 1c and BF 2a according to the invention were tested in comparison thereto.

[0032] The motor vehicle brake fluids used had the following compositions:

[0033] BF 1:

[0034] 55.00% by weight of triethylene glycol methyl ether borate,

[0035] 43.44% by weight of a mixture of triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, triethylene glycol monobutyl ether and diethylene glycol monomethyl ether,

[0036] 1.51% by weight of a mixture of 1,1′-iminodipropan-2-ol, phenothiazine and a phosphoric acid ester

[0037] 0.05% by weight of 1H-1,2,3-tolutriazole

[0038] BF 2:

[0039] 68.00% by weight of triethylene glycol methyl ether borate,

[0040] 28.84% by weight of a mixture of triethylene glycol monomethyl ether, diethylene glycol monomethyl ether, diethylene glycol monobutyl ether and diethylene glycol,

[0041] 3.11% by weight of a mixture of 1,1′-iminodipropan-2-ol, phenothiazine and a phosphonic acid derivative

[0042] 0.05% by weight of 1H-1,2,3-tolutriazole.

[0043] Brake fluids BF 1a and BF 2a according to the invention differ from BF 1 and BF 2 only in that the 0.05% by weight of 1H-1,2,3-tolutriazole has been replaced by the same amount of 1H-1,2,4-triazole; brake fluid BF 1b comprises 0.04% by weight of 1H-1,2,4-triazole and 0.01% by weight of adenine instead of 0.05% by weight of 1H-1,2,3-tolutriazole; brake fluid BF 1c according to the invention comprises 0.07% by weight of 1H-1,2,4-triazole (but only 54.98% by weight of triethylene glycol monomethyl ether borate) instead of 0.05% by weight of 1H-1,2,3-tolutriazole.

[0044] The applicational studies were carried out in accordance with the PSA Peugeot-Citroën D 53 5387 copper corrosion test in the 2000 version. In this test, firstly a copper sheet which has previously been cleaned in acetone, with a total surface area of 1000 cm² (dimensions: 71.4 cm×7 cm×0.05 cm) is coiled up in the form of a round cylinder in such a way that the metal surface in the interior is not in contact with itself. The copper sheet is placed in a mixture of 294 ml of brake fluid containing 6 ml of distilled water and heated at 100° C. for 100 hours; 10 liters per hour of air which had previously been passed through distilled water cooled to 2° C. are passed through the fluid. After the test, the appearance of the unrolled copper sheet and the appearance of the fluid are assessed visually, and the copper content of the fluid is determined.

[0045] The results from the test described are shown in Table 1 below. TABLE 1 Cu content Cu sheet, [mg/kg] appearance Fluid, appearance pH BF 1 24 Thick coating Cloudy, 7.4 (Comp.): precipitations BF 1a: 29 Bright, no Amber-colored, 7.6 coating clear BF 1b: 34 Bright, no Amber-colored, 7.6 coating clear BF 1c: 18 Bright, no Amber-colored, 7.5 coating clear BF 2 10 Thick coating Cloudy, 7.6 (Comp.): precipitations BF 2a: 30 Bright, no Amber-colored, 7.7 coating clear

[0046] It can be seen that the formulations according to the invention, in contrast to conventional brake fluids, such as BF 1 and BF 2, not only offer very good copper corrosion protection with a low Cu content in the test fluid, but in addition also produce a coating-free metal surface and no sediment formation in the brake fluid.

[0047] The results shown below for the corrosion test in accordance with SAE J 1704 (January 1997) also confirm that brake fluids according to the invention, such as BF 2a, having a low 1H-1,2,4-triazole content offer advantages over formulations not in accordance with the invention, such as BF 1d, having a high 1H-1,2,4-triazole content, since they exhibit a better corrosion result in the case of copper (BF 1d corresponded to BF 1, but comprised 0.20% by weight of 1H-1,2,4-triazole instead of 1H-1,2,3-tolutriazole and comprised only 54.85% by weight of triethylene glycol monomethyl ether borate):

[0048] The results from the corrosion test in accordance with SAE J 1704 (January 97) are shown in Table 2 below. TABLE 2 BF 2a BF 1d (comparison) Metal Weight change [mg/cm²] Weight change [mg/cm²] Tinplate 0.00 −0.01 Steel +0.03 0.00 Aluminum 0.00 0.00 Cast iron +0.05 +0.02 Brass 0.00 0.00 Copper −0.05 −0.11 Zinc +0.05 +0.04 

We claim:
 1. A hydraulic fluid having improved corrosion protection, comprising (a) from 0.005 to 0.125% by weight of 1H-1,2,4-triazole, and (b) from 0 to 10% by weight of one or more further corrosion inhibitors, where, on concomitant use of 1H-1,2,3-benzotriazole and/or 1H-1,2,3-tolutriazole and/or derivatives thereof, the weight ratio between 1H-1,2,4-triazole and said 1H-1,2,3-triazoles must be greater than 4:1.
 2. A hydraulic fluid as claimed in claim 1, comprising from 0.005 to 0.08% by weight of component (a).
 3. A hydraulic fluid as claimed in claim 1 or 2, comprising component (a) as the only triazole-based corrosion inhibitor.
 4. A hydraulic fluid as claimed in claim 1 or 2, comprising, as component (b), from 0.001 to 0.5% by weight of one or more compounds from the group consisting of benzimidazole, hydrogenated 1H-1,2,3-tolutriazole, purine, adenine, 6-chloropurine, 2,6-dichloropurine, 6-methoxypurine, 1H-1,2,3-triazolo(4,5-b)pyridine, 6-histaminopurine and 6-furfurylaminopurine, and, if desired, further corrosion inhibitors other than (a).
 5. A brake fluid for motor vehicles, comprising, as corrosion inhibitors, components (a) and, if desired, (b) as claimed in any one of claims 1 to
 4. 6. A brake fluid for motor vehicles as claimed in claim 5, furthermore comprising, besides components (a) and, if desired, (b), from 0.1 to 97% by weight of one or more polyglycol ethers and/or boric acid esters thereof.
 7. A brake fluid for motor vehicles as claimed in claim 5 or 6, furthermore comprising, besides components (a) and, if desired, (b), from 0.1 to 50% by weight of one or more polyglycols. 